Measurement of concentrations of dissolved solvent

ABSTRACT

In a system for measuring minute concentrations of dissolved solids in liquids, seed particles of a known diameter are mixed with the liquid and the mixture of the seed particles and the liquid is atomized into droplets of a known size. The droplets are dried to residue particles comprising the seed particle surrounded by a shell of the dissolved solid. The size of the residue particles are measured by passing the residue particles through a laser beam to scatter light to photodetectors so as to generate a pulse from each particle. The amplitude of the pulses will indicate the size of the residue particle from which the thickness of the shell and the concentration of the dissolved solid can be determined.

BACKGROUND OF THE INVENTION

This invention relates to a system for measuring minute quantities ofsolid materials and, more particularly, to a system for measuring minuteconcentrations of dissolved solids in liquids.

In the manufacture of semiconductor chips, super pure water and othersolvents are needed. For example, in the manufacture of very large scaleintegrated (VLSI) circuits, the surface of the semiconductor wafer whichis to become the VLSI circuit must be repeatedly washed or cleansed.Very pure water is used to wash the surface and any nonvolatile residuedissolved in the water will have a tendency to remain on the surface ofthe wafer when the ultrapure water has evaporated. Since minimal tracesof residue material on the surface of the semiconductor wafer can causedefects in the resulting semiconductor device, it is imperative to usevery pure water of the highest quality to prevent or eliminate possibledefects. There is also a need for very pure water in other industries,such as in the pharmaceutical industry and in electric power generating.Water for pharmaceutical injectibles must be free from bacterial andpyrogens if it is not to cause pathological effects when injected intothe human body. The electric power industry requires ultrapure water forhigh pressure steam generation to drive turbine generators becauseimpurities in the water can be deposited on turbine blades and causeunbalancing of the turbine or cause corrosion. Accordingly, there is aneed to be able to measure minute quantities of dissolved solvents inliquids to ensure that the liquid has the requisite purity for theapplication.

One effective system for measuring minute concentrations of dissolvedsolids and liquids in use prior to the present invention involvesatomizing the liquid into droplets 10 to 100 microns in diameter, thendrying the droplets to a residue in the form of a particle approximatelyspherical in shape. The size of the particle is then measured by passingthe particle through a light beam to scatter light from the beam tophotodetectors or photodetectors. The amplitude of the resulting pulseproduced by the photodetector will provide a measurement of the diameterof the particle. If the size of the original droplet is known, theconcentration of the dissolved solid in the liquid from which thedroplet was formed can be determined.

The concentration of the solid in the liquid is related to the diameterof the residue particle and the droplet diameter in accordance with thefollowing equation:

    C=(d/D).sup.3                                              ( 1)

in which C is the concentration, d is the (1) residue particle diameterand D is the liquid droplet diameter. By making the liquid dropletbigger, a more minute concentration can be detected. However, dropletslarger than 100 microns cannot be easily dried. As a practical matter toensure reliability of the measurement, droplet sizes of around 50microns are employed. If the concentration of the liquid were 1 part permillion and the original droplet size were 50 microns, then the diameterof the residue particle would be about 0.5 microns which corresponds tothe sensitivity of some relatively inexpensive particle detectors. Somehigher quality particle detectors have a sensitivity to detect particles0.3 microns in size. Assuming a 50 micron droplet since, such a particledetector can detect a concentration of (0.3/50)³ or about 200 parts perbillion. This sufficient sensitivity is not sufficient for VLSI circuitsand some other applications. State of the art particle detectors candetect smaller particles and have sufficient sensitivity for VLSIapplication, but such particle detectors are many times for expensivethan the particle detectors with a sensitivity to detect 0.3 micronparticles.

SUMMARY OF THE INVENTION

The purpose of the present invention is to achieve satisfactorysensitivity in the detection of dissolved solids with moderately pricedparticle sensing instruments. In accordance with the present invention,a seed particle of known diameter is added to the atomized dropletbefore it is dried. The diameter of the seed particle is selected to beslightly larger than the sensitivity of the instrument for detectingparticle size. When the atomized droplet is dried, the resulting residueparticle, called a residue measurement particle, will have a diameterwhich is greater than the known diameter of the seed particle. Theamount that the diameter of the residue particle is greater than theseed particle will provide an accurate indication of the amount ofdissolved solid in the liquid with the measurement achieving asubstantial greater sensitivity in the measurement of the concentrationof the dissolved solid for the selected particle measuring. For example,if the original droplet size is 50 microns and the seed particle has adiameter of 0.343 microns, a dissolved solid concentration of about 19parts per billion will cause an increase in the pulse amplitude overthat which would be produced by the seed particle of about 10 percent.Thus, 19 parts per billion is comfortably measured by the instrument ofthe present invention using a particle detector with a sensitivity todetect 0.3 micron particles and without pushing any of the parameters totheir limits. By decreasing the size of the droplets, or by decreasingthe size of the seed particle and using a particle detector with greatersensitivity, much greater sensitivity of the instrument to dissolvedsolids can be obtained. By increasing the size of the atomized droplets,the sensitivity can also be increased.

To make accurate measurements, the rate of flow of the entrainedparticles through the laser beam and the intensity of the laser beammust be kept constant or variations in these parameters accounted forbecause the velocity of the particle passing through the laser beam andthe intensity of the laser beam will affect the amplitude of the pulsesgenerated by the photodetector detecting light scatter from theparticles. In accordance with one embodiment of the present invention,the method of measurement is calibrated for variations in the flow rateor the intensity of the laser beam by forming a second liquid dropletwith a much larger seed particle, called a monitor seed particle, in thedroplet. The resulting residue particle, called a residue monitorparticle, formed from the droplet with the monitor seed particle will becorrespondingly larger. The size of the monitor seed particle isselected so that variations in the concentration of the dissolved solidto be measured will cause only a small variation in the size of theresidue monitor particle. At the upper end of the calibration curvewhich relates the particle size to the pulse amplitude in the laser beaminstrument measuring the particle size, the curve becomes substantiallyflat so that further increases in particle size do not result inincreases in pulse amplitude. The size of the monitor seed particle isselected so that it will fall in this flat portion of the calibrationcurve. Thus, both because the size of the monitor seed particle isrelatively large and because it occurs in the flat portion of thecalibration curve, variations in the concentration of the dissolvedsolid to be measured will have substantially no effect on the pulseamplitude. On the other hand, the pulse amplitude will still varysubstantially with both the flow rate of the air stream entraining theresidue particles and also with the intensity of the laser beam. Morespecifically, a change in the flow rate or a change in the beamintensity will cause the same percentage change in the pulse amplitudefor pulses generated from a residue measurement particle and from aresidue monitor particle. Thus, the variations in the pulse amplitudegenerated by the residue monitor particles can be used to correct themeasurements of the concentration of the dissolved solid generated fromthe smaller residue measurement particles for variations caused bychanges in the flow rate or in the intensity of the laser beam.

In an alternative embodiment of the invention, an atomizer is employedhaving a plurality of chambers behind separate nozzles each capable ofinjecting droplets into a common drying column to dry the droplets. Theliquid sample containing the dissolved solid mixed with the 0.343 micronbeads is drawn through one of the nozzle chambers to be injected intothe drying column. Another chamber in the atomizer receives ultrapureliquid mixed with calibration beads of the same 0.343 size and dropletsfrom this chamber are also injected periodically into the drying column.When these droplets containing the calibration beads are dried and thendetected by the particle size measurement instrument, these measurementsprovide a basis for an updated calibration of the instrument.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 schematically illustrates the method and system of the presentinvention;

FIG. 2 schematically illustrates a portion of the atomizer employed inthe system of the present invention to generate droplets of a preciselycontrolled size;

FIG. 3 schematically illustrates the system for controlling the atomizerto eject droplets in sequence from a plurality of different nozzles;

FIG. 4 schematically illustrates the particle detector employed in thepresent invention;

FIG. 5 is a curve illustrating how the pulse amplitude produced by theparticle detector employed in the system of the present invention varieswith particle size; and

FIG. 6 schematically illustrates an alternative embodiment of thepresent invention using a different calibration technique.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in FIG. 1, a sample of the liquid to be measured from a source11 of the liquid sample is caused to flow into a mixer 13, which alsoreceives spherical seed particles from a reservoir 15 of seed particlessuspended in ultrapure water. The flow of the liquid sample and the seedparticles into the mixer 13 is controlled by valves 16 and 18,respectively. The seed particles should be insoluble in the sampleliquid and when the sample liquid is water, the seed particles, forexample, may be latex beads. In the preferred specific embodiment, mostof the latex beads have a diameter of 0.343 microns. A few of the latexbeads, which are the monitor seed particles, have a diameter of 1.1microns. From the mixer 13, the sample liquid entraining the sphericalbeads is caused to flow through an atomizer 17, which atomizes a portionof the sample stream containing the spherical beads into droplets, whichare controlled to be 50 microns in diameter. A pump 20 is provideddownstream from the atomizer 17 and, by negative pressure or suction,the pump 20 draws the sample from the sample source into the mixer 13,draws the beads from the reservoir 15 into the mixer 13 and draws themixture of the beads and the sample through a chamber in the atomizer17. As the mixture of beads and the liquid sample are drawn through theatomizer chamber, the atomizer atomizes a portion of the sample intodroplets ejecting the droplets downwardly into a drying column 22.

Preferably, the beads are mixed into the liquid at a concentration ofabout 250,000,000 per milliliter so that most of the droplets ejected bythe atomizer will contain one bead. Because of the randomness of thedistribution of the beads in the liquid, some of the droplets ejected bythe atomizer will have multiple beads and some of the droplets will haveno beads, but, for the most part, each droplet will contain one latexbead. Preferably, the atomizer 17 is of the bubble jet type which isused in laser jet printers, but it also may be a vibrating orificenozzle of the type provided in the equipment manufactured by TSI, Inc.or such as manufactured by Sonotec Corporation. Other types of atomizerswhich are known in the art and which would be suitable for use in thisinvention are the compressed air atomizer, the spinning disc atomizerand the spinning top atomizer.

FIG. 2 is a sectional view schematically illustrating a portion of theatomizer showing one nozzle opening of the atomizer in section. As shownin FIG. 2, the mixture of the sample and the latex beads are drawnthrough a chamber 23 in the atomizer by pump 20. Nozzle opening 25 opensto the chamber 23 and extends vertically from the chamber 23 opening tothe top of the drying column 22. A resistor 27 is positioned in thechamber 23 over the nozzle opening 25. To eject a droplet, a high energyelectrical pulse is applied to the resistor 27, which becomes heated aconverts a portion of the sample in the chamber 23 to a steam bubble.The pressure of the steam bubble ejects a droplet of the sample throughthe nozzle opening 25 and into the drying column 22. With this kind ofnozzle, the diameter of the ejected droplet can be precisely controlled.Because the atomizer employed is identical to that used in printers,i.e., the Hewlett Packard Think Jet printer, the atomizer iscommercially available at a low price. A typical low priced atomizer has30 nozzle openings with a resistor in the chamber behind each nozzleopening. In accordance with the preferred embodiment of the presentinvention, four of these nozzle openings are used. The resistor behindthe four selected nozzles are pulsed in sequence to cause droplets to beejected from the four nozzle openings in sequence. A plurality of thenozzle openings are used in this manner to enable the atomizer tocontinue to be used should one of the nozzle openings become clogged.

FIG. 3 schematically illustrates the nozzle sequencing system of thepresent invention. As shown in FIG. 3, a sequential energizer 29 appliespulses in sequences to four resistors 27 behind four of the nozzleopenings in the atomizer 17 thus causing droplets to be ejected from thefour nozzle openings in sequence. The bubble jet type atomizer nozzlefor printing applications is described in Output Hardware Devices,Chapter 13, by William J. Lloyd and Howard H. Taub, published byAcademic Press in 1984.

The drying column 22 is a cylindrical chamber with its axis positionedvertically. The column 22 serves the function of drying the droplets toresidue particles, the function of neutralizing the static charges onthe droplets, and the function of separating any smaller satellitedroplets produced by the atomizer from the droplets having the preciselycontrolled size of 50 microns in diameter. Ionized drying air isintroduced into the column 22 about a third of the way from the bottomof the column and drying air flows upwardly and downwardly from itspoint of introduction into the column. The ionized air contains bothpositive and negative charges and, as it passes over the droplets, itneutralizes any static electrical charge on the droplets.

In addition, the air in the column flowing upwardly from the point ofintroduction will separate smaller satellite droplets in the column fromthe 50 micron droplets and carry the satellite droplets up to an exhaustport near the top of the dryer column where the upwardly flowing aircarrying the satellite droplets is exhausted from the column through avalve 26. The 50 micron droplets are injected into the column by theatomizer 17 with downward momentum and the upwardly flowing drying airin the column is unable to overcome the momentum of the 50 microndroplets. Accordingly, the 50 micron droplets pass through the upwardlyflowing air to the lower part of the column. The satellite droplets, onthe other hand, being lighter in weight than the 50 micron diameterdroplets have insufficient momentum to overcome the upwardly flowingdrying air in the upper part of the column and become entrained in thisupwardly flowing air and are carried out through the exhaust port 24.

As the droplets pass through the upwardly flowing air and then throughdownwardly flowing air in the lower part of the column, the droplets aredried to their residue, which, in the case of most droplets, will be aspherical bead surrounded by a spherical shell of the dissolved solid inthe sample liquid. The downward flow of air in the column 22 carries thedried residue particles out of an exhaust port 28 in the bottom of thecolumn.

The drying air is supplied from a pressured source 31 through aregulating valve 32, a desiccant air dryer 34, and a filter 35 to anionizing chamber 36. In the ionizing chamber 36, high voltage pulsesalternating in polarity are applied to the air between electrodes 38 and40 from a high voltage supply 42. The high voltage source 42 applieshigh voltage pulses between the electrodes 38 and 40 in the range of4,000 to 10,000 volts at a frequency ranging from 0.5 to 5 cycles persecond with the pulse polarity alternating on successive pulses. Thehigh voltage applied to the air ionizes the air and the ionized airflows to an air heater 44. Heated air from the air heater 44 is directedinto the ionized air inlet of the column 22. The ionized air will carrypositive and negative charges on the individual air ions and thesepositive and negative charges will neutralize any static electricitycharges on the droplets produced by the atomizer 17.

The ionizer comprising the ionizing chamber 36 and the high voltagesupply 42 may be similar to that supplied by Static Control Servicesunder the trademark PULSEBAR® and as described in the Static ControlServices catalog.

The aerosol of the residue particles passing through the exhaust port 28at the bottom of the drying column 22 enters a mixer 46 where theaerosol is mixed with diluting air supplied through a filter 48 from asource of pressurized air 49. The diluted aerosol then flows through alaser beam particle measuring instrument, which may be of the typemanufactured by the Hiac Royco Division of Pacific Scientific Companyand identified by Model No. 5230A. This particle measuring instrumenthas a sensitivity to detect particles down to 0.3 microns in size. Theprinciple of operation of the particle measurement instrument isdisclosed in U.S. Pat. No. 4,842,406 to Kenneth Paul VonBargen and inU.S. Pat. No. 4,984,889 to Holger T. Sommer. The instrument comprises alaser particle detector 50 in which the residue particles pass through alaser beam and a pulse amplitude analyzer 52, which measures theamplitude of each pulse. The atomizer 17 ejects droplets at a rate suchthat one particle at a time passes through the laser beam. As shownschematically in FIG. 4, the aerosol of residue particles is directed bynozzles 53 through a laser beam generated by a laser beam source 54. Aseach particle passes through the laser beam, it will scatter light to aphotodetector 55 and the amplitude of the resulting pulse will be anindication of the size of the particle causing the pulse. The pulsesproduced by the photodetector are applied to the pulse amplitudeanalyzer 52 which measures the amplitude of each pulse produced by thephotodetector. Each pulse amplitude is measured by determining anamplitude range into which the pulse falls and a counter is provided tocount the number of pulses in each range. In this manner, the pulseamplitude of each pulse produced by a residue particle containing a0.343 micron bead is measured and the amplitude of each pulse producedby a particle containing a 1.1 micron bead is also measured. Theamplitude range into which pulses produced from the particles containingthe 0.343 micron beads represents a corresponding size range ofparticles and represents a measurement of particle size. Particlesconsisting entirely of the dissolved solid residue will be much smallerand any pulses caused by these particles will be disregarded. Inaddition, pulses caused by particles containing more than one bead willbe much larger and can be disregarded. The amplitude of pulses caused byresidue particles each containing one 0.343 micron bead will provide anaccurate measurement of the concentration of the dissolved solid in thesample liquid to about 19 parts per billion.

The Model 5230A particle measuring instrument, manufactured by the HiacRoyco Division of Pacific Scientific Company, has a sensitivity tomeasure particles down to 0.3 microns. This instrument has asignal-to-noise ratio of 2.76 for pulses produced in response to 0.343micron particles, which means that this instrument is quite practical tomeasure sizes of particles on this order of magnitude. With thisparticle measuring instrument, a concentration of dissolved solids inthe original liquid of 19 parts per billion can easily be detected as isexplained below.

The size of a residue measurement particle can be represented by thefollowing expression:

    D.sub.grown =(1+m)D.sub.seed                               (2)

wherein D_(grown) is the diameter of the residue measurement particle,D_(seed) is the diameter of the measurement seed particle or 0.343microns and m is the increase in the diameter of the residue particleover the seed particle. The ratio of the signal for the residue particleto the signal for the seed particle can be expressed as follows:##EQU1##

In expressions (3) and (4), V_(grown) is the output pulse amplitude ofthe particle detector in response to the residue particle with a shellof dissolved solid, V_(seed) is the output pulse amplitude of theparticle detector that would be produced in response to the seedparticle with no shell. The exponent n is a value depending upon thesize of the particle ranging between 5 and 6.

Since m is very small, the expression for the ratio of the signals canbe approximated as follows:

    V.sub.grown /V.sub.seed =1+mn                              (5)

Solving equation (5) for m, the following equation results:

    m=1/n [V.sub.grown /V.sub.seen -1]                         (6)

For a seed particle size of 0.343 microns, the value of n is about 5.12.If V_(grown) is about 10 percent greater than V_(seed), then solvingequation (6) for m results in the following:

    m=[1/5.12]×[1.1-1]=0.0195                            (8)

In other words, the 10 percent larger signal is produced as a result ofthe fact that D_(grown) is 1.95 percent larger than D_(seed). The volumeof the solid residue formed in the shell around the seed particle can becalculated as follows. The shell thickness t is: ##EQU2## The volume ofthe shell is the surface area multiplied by the shell thickness t. Theseed particle surface area is:

    A=π×(D.sub.seed).sup.2                            (11)

Accordingly, the volume V of the shell is: ##EQU3## Substituting for m,the volume of the shell is

    V=π/2×(D.sub.seed).sup.3 [(1/n)×(V.sub.grown /V.sub.seed -1)](14)

The concentration of the solid C can be expressed in terms of the volumeof the residue shell V_(shell) and the volume of the droplet V_(droplet)as follows: ##EQU4## in which D_(droplet) is the diameter of the dropletor, in other words, in the preferred embodiment, 50 microns.Substituting the known values for equation 17, the following valuesresult: ##EQU5## or 19 parts per billion. Thus, a 10 percent change inthe voltage signal from the particle detector will result from aconcentration of 19 parts per billion. In the Hiac Royco 5230Ainstrument, the 0.343 micron particle will produce a 30 millivolt outputsignal so a 10 percent increase corresponds to 3 millivolts of signal.Three millivolts provides a signal-to-noise ratio well over 2 and,therefore, is easily detectable.

By a similar calculation, it can be shown that if a particle detectorhaving the sensitivity to detect 0.1 micron particle and a seed particleof this size were used, then dissolved concentrations of less than 1part per billion could be detected.

To provide continuous dynamic calibration of the system, monitor seedparticles, much larger than measurement seed particles are mixed withthe measurement seed particles in the seed particle reservoir 15 in aratio of about one monitor seed particle to 100 measurement seedparticles. The monitor seed particles are also latex beads, and, in apreferred embodiment, have a diameter of 1.1 microns compared to the0.343 micron diameter measurement seed particles. The monitor seedparticles are drawn into the mixer 13 along with the measurement seedparticles and some of the droplets ejected by the nozzle will containthe monitor seed particles. While the residue particles produced fromthe droplets containing monitor seed particles would have some variationin size due to changes in the concentration in the dissolved solids inthe sample liquid, this variation in size will have negligible effectbecause of the size of the monitor seed particle being 1.1 microns indiameter and also because a particle of this size causes the signalvoltage produced by the particle detector 23 in response to the particleto occur on a flat portion of the calibration curve of the particlemeasuring instrument where changes in particle size cause little changein the output voltage. FIG. 5 illustrates the calibration curve for theparticle detector plotting the voltage of the output pulses produced bythe particle detector verses the particle diameter in microns on alogarithmic scale. In FIG. 5, the variation in the pulse amplitude withparticle diameter becomes substantially flat above 1 micron.

The voltage amplitude produced from the first monitor seed particle isstored in the instrument as a benchmark voltage to provide a basis forcorrections to avoid errors due to drift in the intensity of the laserbeam of the particle detector 23 or in the flow rate of the particlesthrough the particle detector 23. A change in either the intensity orthe flow rate of the particle will cause a corresponding change in thesignal voltage. To prevent changes in the flow rate or in the intensityof the laser beam from causing errors in the concentration measurement,measurement is calibrated in accordance with the voltage amplitudecurrently being generated from the residue monitor particles producedfrom droplets containing monitor seed particles. The ratio of thisvoltage to the benchmark voltage which has been stored in the instrumentis then used to correct the output voltage V_(grown) generated from ameasurement residue particle in accordance with the following formula:

    V".sub.grown =V.sub.grown ×(V.sub.o /V.sub.mon)      (19)

in which V'_(grown) is the output voltage of the particle detectorcorrected for drift in the flow rate and/or laser beam intensity,V_(grown) is the current output voltage of the particle detector,generated from a measurement residue particle produced from a dropletcontaining a measurement seed particle. V_(o) is the original signalgenerated by the first residue monitor particle produced from a dropletcontaining a monitor seed particle and stored as a benchmark voltage andV_(mon) is the current signal voltage generated from the residue monitorparticle obtained from a droplet containing a monitor seed particle.Thus, by using the monitor particles and equation 19, the measurementsof the dissolved solids can be corrected for drift in the intensity ofthe laser beam in the particle detector or for drift in the flow rate ofparticles through the laser beam.

FIG. 6 illustrates an alternative embodiment of the present inventionemploying an alternative system for calibrating the measurement. Theembodiment of FIG. 6 is like that of FIG. 1 except that the systememploys an atomizer 61 which, instead of being the same atomizer that isemployed in a monochromatic ink jet printer, is the atomizer that isemployed in a color printer. In this atomizer, separate chambers 62 and64 are provided behind separate nozzles in the atomizer so that separatesources of liquid to be atomized can be directed through differentnozzles. As shown in FIG. 6, chamber 62 receives the mixture of liquidsample with the 0.343 micron diameter beads from the mixer 13. Acalibration bead reservoir 63 containing beads 0.343 microns in diameterin ultrapure water is connected to the second chamber 64 in the atomizerthrough a valve 65. The chambers 62 and 64 are located over differentnozzles in the atomizer 17 and both of the chambers are connecteddownstream to the pump 20. When the valve 65 is open, beads mixed inultrapure water will be drawn from the reservoir 63 through chamber 64and ultrapure water droplets containing the calibration beads can beejected into the drying column 22 by applying pulses to the resistorsover the nozzle openings connected to the chamber 64. Each chamber inthe atomizer 61 will have eight nozzles communicating with the dryingcolumn 22. The system of the present invention uses four nozzles foreach chamber operated in sequence as desired in connection with thenozzle 17 in the embodiment of FIG. 1. In this manner, ultrapure waterdroplets containing the 0.343 micron calibration beads can beselectively ejected into the drying column. These droplets will be driedand the resulting residue particles will consist entirely of thecalibration beads 0.343 microns in diameter. Accordingly, when thesecalibration beads pass through the particle detector 50, the resultingamplitude measurements obtained by the pulse analyzer provides a currentcalibration voltage representing the size of the measurement beadagainst which the voltage from the residue particle containing thedissolved solid shell can be directly compared to obtain an accuratesize measurement.

As described above, a known amount of added material is provided to eachdroplet very conveniently by providing the bead of a known diameter ineach droplet. However, the same effect can be achieved by mixing in theknown amount per unit volume of a dissolvable material in the liquidbefore the liquid is transported to the atomizer for being atomized intodroplets. In this latter arrangement, the resulting residue particleswill not consist of a bead with a surrounding shell of the dissolvedsolid material. Nevertheless, the increase in size of the residueparticles over that which would result from the added material alonewill be the same as that for when the added material was in the form ofbeads.

This and other modifications may be made to the above-described specificembodiments of the invention without departing from the spirit and scopeof the invention which is defined in the appended claims.

I claim:
 1. A method of measuring minute concentrations of a soliddissolved in a liquid comprising mixing a known amount of seed materialinto said liquid, atomizing said liquid into droplets including ameasurement droplet of a known size containing a known amount of saidseed material, drying said measurement droplet to a residue particlecontaining said solid and said seed material in solid form, andmeasuring the size of said residue particle.
 2. A method of measuringminute concentrations of dissolved solids in a liquid comprising mixinga solid seed particle of a known size with said liquid, atomizing saidliquid into droplets including a measurement droplet of a known sizecontaining said seed particle, drying said measurement droplet to aresidue particle consisting said seed particle and a shell of said solidmaterial around said seed particle, and measuring the size of saidresidue particle.
 3. A method of measuring minute concentrations of asolid dissolved in a liquid as recited in claim 2, wherein said step ofmeasuring the size of said residue particle comprises passing saidresidue particle through a light beam to scatter light from saidparticle, detecting light scattered from said particle to generate apulse and measuring the amplitude of said pulse.
 4. A method as recitedin claim 2, further comprising mixing a multiplicity of solid seedparticles of known size with said liquid prior to atomizing said liquidinto said droplets so that a multiplicity of said droplets contain seedparticles, drying said multiplicity of droplets to a multiplicity ofresidue particles, and measuring the size of the particles of themultiplicity of residue particles.
 5. A method as recited in claim 4,wherein the step of measuring the size of said residue particlescomprises passing said residue particles through a light beam one at atime to scatter light from each of said particles, and detecting lightscattered from each of said particles to generate a pulse, and measuringthe amplitude of each pulse.
 6. A method as recited in claim 5, whereina first set of said seed particles have a first predetermined size and asecond set of said particles have a second predetermined sizesubstantially larger than said first predetermined size, the size ofsaid second set of particles being selected so that the amplitude ofpulses produced from residue particles including seed particles of saidsecond set do not vary substantially with changes in the concentrationof dissolved solids in said liquid.
 7. A method as recited in claim 6,wherein the seed particles of said second set have a diameter of about1.1 microns.
 8. A method as recited in claim 5, further comprisingatomizing an ultrapure liquid containing a multiplicity of solidcalibration seed particles of known size into calibration droplets,drying said calibration droplets to residue calibration particlesconsisting of said calibration seed particles, passing said calibrationresidue particles through said light beam to scatter light from saidcalibration residue particles, detecting light scattered from saidcalibration residue particles to generate calibration pulses andmeasuring the amplitude of calibration pulses.
 9. A method as recited inclaim 4, wherein said droplets are dried by injecting said droplets intoa column in a direction to pass axially through said column and passingdrying air over said droplets in said column in a reverse direction tothe direction of movement of said droplets in said column to separatesmaller satellite droplets from said droplets of a constant size.
 10. Amethod as recited in claim 9, wherein said drying air is ionized toneutralize static charges on said droplets.
 11. A method as recited inclaim 2, further comprising atomizing an ultrapure liquid containing asecond solid seed particle of a known size into droplets including acalibration droplet containing said second seed particle, drying saidcalibration droplet to a second residue particle consisting of saidsecond seed particle and measuring the size of said second residueparticle.
 12. An apparatus for measuring minute quantities of a soliddissolved in a liquid comprising means to mix solid seed particles of aknown size with said liquid, atomizing means to atomize the mixture ofsaid liquid and said seed particles into droplets of a known sizewherein some of said droplets contain a single seed particle, means todry said droplets to residue particles comprising said seed particlessurrounded by a shell of said solid material, means to measure the sizeof said residue particles.
 13. An apparatus as recited in claim 12,wherein said means to atomize the mixed liquid and particles intodroplets comprises means defining a nozzle aperture and a chamber behindsaid aperture to receive said mixed liquid and particles and means togenerate a bubble in said chamber behind said nozzle to eject a dropletout of said nozzle.
 14. An apparatus for measuring dissolved solids in aliquid as recited in claim 12, wherein said means to dry said dropletsto residue particles comprises a column, said atomizing means ejectingsaid droplets into said column to pass axially through said column in afirst direction, means to introduce drying gas into said column to passover said droplets in the reverse direction from said first direction.15. An apparatus for measuring dissolved solids in a liquid as recitedin claim 14, wherein said drying gas is introduced into said columnmidway between its ends to travel through said column in said reversedirection in part of said column and in said first direction in part ofsaid column to carry said residue particles out of said column.
 16. Anapparatus for measuring minute dissolved solids in a liquid as recitedin claim 14, further comprising means to ionize said gas prior tointroducing said gas in said column.
 17. An apparatus as recited inclaim 12, wherein said atomizing means comprises a plurality of nozzleseach communicating with a separate chamber in said atomizing means,means to introduce said mixture of said liquid and seed particles into afirst one of said chambers to be ejected in droplets from the nozzlecommunicating with said first one of said chambers, and means tointroduce into a second one of said chambers ultrapure liquid containingsolid calibration seed particles of a known size, said atomizer havingmeans to generate a bubble in said ultrapure liquid behind the saidsecond nozzle to eject a calibration droplet of said ultrapure liquidthrough said second nozzle opening, said drying means operating to drysaid calibration droplets to calibration residue particles, said meansto measure the size of said particles comprising means to measure thesize of said calibration residue particles.
 18. An apparatus as recitedin claim 17, wherein said measuring means comprises a laser beam andmeans to direct said residue particles including said calibrationresidue particles through said laser beam one at a time to scatter lightfrom said residue particles, photoelectric means to detect the lightscattered from said residue particles to generate a pulse in response toeach residue particle and means to measure the amplitude of each pulsegenerated by said photoelectric means.